Create integration tests

[aik099]

For integration tests need to create class, that would:

• implement ClientInterface
• return stubbed responses based on API docs https://docs.atlassian.com/jira/REST/latest/

I have no idea right now how to test actual ClientInterface implementations though.

Actually we might create mini JIRA REST API endpoint that would return stubbed responses instead. Then we can run all tests through each client (CurlClient and PHPClient) to see if they actually works.

[aik099]

Decided to take different route. In #108 we have integration tests for ClientInterface implementing classes. Then in #114, since we know that client classes work as expected, the mock for ClientInterface is created and we test Api class.